ROBUSTA is the first French university CubeSat designed with the support of CNES. The objective of the payload experiment is to measure the radiation induced degradation of electronic devices. Flight data will be compared to the results of a novel prediction method taking into account the Enhanced Low Dose Rate Sensitivity. The second interesting point of this project is that it's a real educational project. 1)2)3)

ROBUSTA is the joint response of five schools of the University Montpellier 2 (UM2), located in southern France, to the 'call for proposal' for student projects in the spatial field, EXPRESSO(EXpérimentations et PRojets Etudiants dans le domaine des SystèmeS Orbitaux), issued by CNES (Centre National d’Etudes Spatiales), the French Space Agency. This program initiative, started in 2006, strives to serve as an educational platform, promoting space and science and attempting to involve students in all aspects of a project from project management to “hands-on” development. The first call of EXPRESSO ideas has permitted to select three projects (out of 8 submitted), among them is the ROBUSTA project lead by the University of Montpellier. 4)5)6)

Background: Within this context, CNES has decided in the meantime to continue the experience with a future more ambitious initiative named RISTRETTO in the framework of the EXPRESSO program. RISTRETTO (Réseau International de SysTèmes oRbitaux ETudiants basés sur une Technique de développement en Open source) is the French acronym for “International Network of Students satellite Projects based on Technical Development in open source.” The overall idea is to organize the international student community around a common project: studying, developing and using new concepts of student satellites.

The RISTRETTO proposal aims to extend the standard CubeSat concept (10 cm sidelength, and ≤ 1 kg mass) to a larger size of satellites of 30 cm sidelength, 30 W, and 30 kg of mass - to enlarge the system capability to handle embedded targets. Its main characteristics are: 7)8)

• Compatibility with all kinds of launchers by the way of a standard interfaces

• Low-cost and reduced time of development using COTS components

• Studied and developed via an OPEN SOURCE concept by an international student community.

• A RISTRETTO goal is also to be an integral part of the GENSO (Global Educational Network for Satellite Operations) initiative. GENSO, an initiative of ESA and ISEB (International Space Education Board), aims to increase the return from educational space missions by forming a worldwide network of ground stations and spacecraft. This will fundamentally change the way that these missions are managed, dramatically increasing the level of access to orbital educational spacecraft. The design and implementation work is being carried out by a distributed set of student and radio amateur teams worldwide and with over 80 educational spacecraft currently planned there is a very large demand for such a project.

ROBUSTA spacecraft:

The proposed CubeSat picosatellite ROBUSTA is entirely built by students of the University Montpellier 2 (UM2) and its various departments (EEA of the Sciences Faculty, ERII of Polytech'Montpellier, GMP and GEII of the IUT de Nîmes) - along with support from CNES and industrial sponsors (Intersil, Texas Instruments, Trad, Cofidur and Farnell).

On a historical note, students of the University of Montpellier were already engaged in 2000 in the cooperative CubeSat project SACRED of the University of Arizona. At the time, TAS (formerly Alcatel Space) acquired the SACRED payload from the University of Arizona. The objective of the project was to study radiation effects on MOS power devices. The payload was entirely designed and built by students of the University of Montpellier. The payload was delivered on time. Montpellier students were sent to Arizona to participate in the assembly and testing of the satellite. SACRED passed all the qualification tests. Unfortunately, the DNEPR-1 launch vehicle failed and SACRED was lost on July 26, 2006 (along with many other small satellites).

The ROBUSTA picosatellite conforms to the CubeSat standard with regard to size (10 cm sidelength), mass (≤ 1 kg) and maximum power consumption (1 W). The project is jointly led by the University of Montpellier and CNES. The goal is to reach a mission time of up to three years.

The basic design is the CubeSat is split into the following parts: Overall management, mechanical structure, power supply and power management subsystem, radio communication subsystem, controller subsystem, payload, ground station, public communication and value of the project.

The bus structure comprises the metallic cube (aluminum panels), and four motherboards on which all subsystems are being mounted. Connectors permit the electrical and mechanical links between the power subsystem and the motherboard. The satellite uses two switches for the separation of Cubesats when ejected by the P-POD (Poly-Picosatellite Orbital Deployer). An example of a motherboard is shown in Figure 3. 9)10)

An inspection of the motherboard configuration (Figure 3, left) reveals an open space in the middle of the board which is reserved for the battery. The reverse side of the motherboard (Figure 3, right) shows the electrical wiring.

Figure 4: CAD model of the ROBUSTA CubeSat with 2 DUTs under the hole in the metallic structure (image credit: UM2)

EPS (Electrical Power Subsystem): The EPS provides the power to all subsystems (Figure 5). The management of power is probably the key point of the satellite. The energy available, 1 W does not make it possible to supply all of the four subsystems at the same time. The power board also comprises the system for supplying the nichrome (NiCr) wire meant to act in the deployment of the antenna.

The solar cells are surface-mounted on the six panels of the CubeSat. Two solar cells are mounted in series on each panel. The solar cells are of the type triple junction GaInP/GaAs/Ge on a Ge substrate from Azur Space. A Li-ion battery from SAFT (France) is used. The nominal energy is 10 Wh; the nominal operational voltage for the subsystems is 3.5 V at 20ºC with 0.5 A. However, the voltage for the RF communications subsystem requires 7.5 V.

Controller subsystem: The objective is the monitor and control all on-board subsystem tasks and to optimize the power consumption. All experiment measurements are being stored in the controller subsystem prior to transmission to the ground. The controller subsystem includes the following elements: a CAN (Controller Area Network) bus, a PIC 18F4680 (Peripheral Interface Controller), a real-time clock, an EEPROM and an anti-latch circuitry. The real-time clock is independent from the microcontroller. It features a precision of 10 s over 4 days. It includes a date in the measured data stream and on the events stored in the journal of events.

The PIC microcontroller communications with all subsystems via the CAN bus, it controls switches and temperature sensors with a SPI (Serial Peripheral Interface) bus and collects measurement data with the help of integrated analog-digital converters.

RF communication subsystem: A UHF/VHF (435.325 MHz/145.95 MHz) transmitter/receiver system is being used with the AX-25 protocol and AFSK (Audio Frequency Shift Keying) modulation. The subsystem comprises five parts: the emission part, the reception part, the PIC board, the general reset part, and the appropriate antennas (2 dipole antennas). 11)

The main ground station is located on the Montpellier University campus using low cost amateur radio hardware. It is meant to send commands or receive data. It will store the received data and processes them. - In Feb. 2009, the ROBUSTA project joined the GENSO (Global Educational Network for Satellite Operations) initiative.

Launch: The ROBUSTA CubeSat was launched on Feb. 13, 2012 as a secondary payload on the maiden flight of the Vega launch vehicle of ASI and ESA. The launch site was Kourou in French Guiana. 12)13)

The multiple payload launch encompasses a primary payload of 400 kg called LARES (LAser RElativity Satellite), and CubeSats (educational payloads) as secondary payloads, whose launch is sponsored by ESA. The free launch of CubeSats was offered by the ESA Education Office in Oct. 2007 (Announcement Opportunity) in cooperation with the Vega program. 14)

CubeSat passenger payloads: Although ESA's Education Office is providing 9 CubeSat positions on the maiden flight of Vega, only 7 CubeSats are confirmed as of December 2011 (Ref. 15). Not all universities that were preselected for the launch opportunity in June 2008, were able to deliver their CubeSat and the requested documentation. Other CubeSat projects, like SwissCube and HiNCube, decided to be launched on commercial flights.

• Xatcobeo (a collaboration of the University of Vigo and INTA, Spain): a mission to demonstrate software-defined radio and solar panel deployment

• e-st@r (Politecnico di Torino, Italy): demonstration of an active 3-axis Attitude Determination and Control system including an inertial measurement unit

• Goliat (University of Bucharest, Romania): imaging of the Earth surface using a digital camera and in-situ measurement of radiation dose and micrometeoroid flux

• PW-Sat (Warsaw University of Technology, Poland): a mission to test a deployable atmospheric drag augmentation device for de-orbiting CubeSats

• MaSat-1 (Budapest University of Technology and Economics, Hungary): a mission to demonstrate various spacecraft avionics, including a power conditioning system, transceiver and on-board data handling.

• UniCubeSat GG (Universitá di Roma ‘La Sapienza’, Italy): the main mission payload concerns the study of the gravity gradient (GG) enhanced by the presence of a deployable boom.

• The Robusta project performed an extensive EMC (Electromagnetic Compatibility) analysis after the communications failure of the CubeSat to study the different high frequency signals emitted during normal operation by the PB (Power Board) and radiocommunication board. Both of them were amongst the most susceptible of all boards to radiate a high frequency field. The most radiating zones were thus determined. - In a second step, an electromagnetic interference was injected without contact on these boards, to make sure the different parts of the satellite will not disturb each other.

In the first analysis, it turned out that the different electronic boards would not disturb each other. However, a high frequency EMI (Electromagnetic Interference) can disturb the supply voltage generation of the PB. Then, the project team injected an EMI on the Rx board, to observe the effect of a parasitic signal captured by the antenna on the reception system. This study has shown that a parasitic interference might disturb the decoding process. Nevertheless, the frequency of the parasitic interference must be really close to the wave carrier to produce demodulation errors. 17)

• The Robusta satellite emitted a weak signal at the beginning of the mission, but no further communication has been received. Investigations identified a fabrication defect which prevents the CubeSat’s batteries from being charged. Although the team is disappointed by the failure, they are planning to use the experience built up with Robusta for the development of future picosatellites. 18)

• On Feb. 16, 2012, the first signal was received from Robusta by the Montpellier ground station. 19)20)

Sensor complement:

Background: The past long-term collaboration between the radiation effects group from the university research lab [IES (Institut d'Electronique du Sud)] and from CNES, established with the SACRED project, provided some very interesting ideas for science applications. As a consequence, the Montpellier team decided to respond to the EXPRESSO 'call for proposal', with a picosatellite carrying a payload devoted to the study of one of the major current concerns for hardness assurance on bipolar technologies, referred to as ELDRS (Enhanced Low Dose Rate Sensitivity).

ELDRS (Enhanced Low Dose Rate Sensitivity):

The objective of the sensor complement is to expose bipolar components to ionizing radiations. The deterioration of the tension currents that result from this will be measured and sent to the ground station for analysis. They will then be compared the the results obtained through ground tests norms. This should enable to validate this test method.

To realize this objective bipolar components, operational amplifiers LM124 and comparator LM1394, will be submitted to the dose flow on-board the satellite.

• The command part (yellow) comprises the microcontroller and switches.

• The metrology part (green) comprises the temperature sensors, plus an OSL (Optically Stimulated Luminescence) dosimeter conceived in the Radiac group from the IES research laboratory, and finally the measurement systems for currents and voltages.

• The experiment part features a blue background. The DUT (Devices Under Test) are two LM124 (4 operational amplifiers per DIL14 package) and two LM139 (4 voltage comparators per DIL14 package). The DUTs are placed beneath a 60 mm x 12 mm slot in the metallic shielding (Figure 4). In this way, the DUTs are in direct contact with the radiative environment. 21)

The environmental metrology will be performed with the help of an OSL dosimeter and two temperature sensors. The instrumentation is of Carmen-2 heritage of CNES flown on Jason-2 (launch June 20, 2008).

A microcontroller collects all data issued from the OSL, from the temperature sensors and from measurements issued from the DUTs. It also manages the communication with the controller subsystem.

Figure 8: Variations of the dose function of time behind shielding of 3 different thicknesses (image credit: CNES)

Ground segment:

The dedicated ground segment is mounted on the roof of the research lab IES (Institut d'Electronique du Sud) building located on the campus of UM2 (University Montpellier 2). The design of this ground station includes as much as possible standard commercial components. However, some parts are custom made mainly to reduce costs or when it becomes too difficult to make them with sufficient operating performances or accuracy. 22)

Provision of communications support over the amateur VHF band for telecommands (uplink) and the UHF band (for downlink). The uplink frequency: = 145.95 MHz, downlink frequency = 437.325 MHz. The data transmission uses the standard AX25 radio-amateur protocol.

Figure 9: Overview of the ROBUSTA ground system (image credit: UM2)

The ROBUSTA ground station is designed for future GENSO (Global Educational Network for Satellite Operation) use. GENSO is a student project of the European Space Agency (ESA) and is coordinated by ESA’s Education Office. To be compatible with this network, the ROBUSTA ground station has been equipped with specific devices like the transceiver or the motor controller.

The information compiled and edited in this article was provided byHerbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.